RT-707 Hitachi transistor replacements

[soundmig]

I've been working on my RT-707 the past few weeks. I replaced the tape counter belt as we probably all do. I also have cleaned all of the top row of switches and the speed control pot. Additionally, I have air blasted (gently) the boards to clean them up and vacuumed out the chassis. I've also adjusted the meter level pots (as per service manual) as well as the playback levels for both FWD and REV playback heads. I am awaiting arrival of an alignment tape to tweak these further and to get them to specification. Playback sounds very good for the most part. I did have some speed wobbles here and there, so I cleaned the white multi-pin connectors as described in another AK thread on speed issues. This seems to have remedied that situation.

My inputs were pretty noisy and a bit crackled sounding, so as other's on AK have suggested, I replaced the C1344 Hitachi transistors on the mic/line board. This made a difference, but there was still some noise. So, I also replaced the A672 transistors. This seemed to make the inputs nice and quite and stable with no crackles. Test tones sound very solid and very steady when listening in "source" mode.

I tried to record the other night and the left channel was barely working (intermittent) and highly distorted in "tape" listen position. The right channel worked better but also has some odd amplitude variations and it sounded slightly distorted when listening to what I was recording in "tape" position. Both channels sounded great in "source" position. As a result I replaced the C1344 Hitachi transistors on the board that supplies signal to the tape heads (RWR-053). That made a pretty substantial difference as both channels now sound "ok" when listening to what is being recorded using the "tape" position. But ..... both channels still have wavering signal levels (of about 1 -2 dB on a steady test tone) and some crackle (only on "tape" position and only on signals I have recorded). So, I'm thinking that the one pair of Hitachi's left in the machine probably need to go. They are 2SC1213 transistors. I'm not sure what to use to replace them. One thread that I found suggested KSC2383. Does anyone know if the KSC2383 is a suitable replacement for the 2SC1213 in this application?

 

[GPS16]

Hi,
when I was the owner of a 707 any levels problems and crackles were due to switch problems and not transistors. Make sure you have not got any switch problems or you will be exchanging transistors for ever!!! Pay particular attention to the rec select switches, they affect the Bias and erase heads. The Equalisation and Bias buttons. The speed button. Also with a dummy tape on the recorder and set to record. Turn the rec channel selectors on and off to exercise the Record Relays. There are delays involved with them to stop residual magnetisation of the erase/record heads, so you have to wait until they have changed state before toggling the switches again.

From my experience, keeping the switches clean was sufficient. This is not an Akai!!

GPS16

 

[soundmig]

Ok, thanks. I've cleaned the record channel select buttons and bias and EQ switches pretty thoroughly, but I haven't slowly turned them on and off to "work" the relays. Thanks for the tip. I'll give that a shot.
Replacing the 1344's took the left channel from being non-functional to functional and cleaned up the crackles on the right channel. Both of the 1344's had black residue all the way down the leads (or legs) so replacement probably was in order?

 

[soundmig]

So, I replaced the Hitachi 1213 transistors on the RWR-053 board. Now the left channel is not recording again, so there is something else wrong in the chain that gets signal to the record heads on the left channel. Replacing the 1213's with 2383's did fix up the right channel real well as it is strong and steady now in record mode listening off of the tape.
I did get an alignment tape. Azimuth was a touch off on both playback heads, but is now spot on. I was surprised by how high the EQ pots had to be turned up (attenuation reduced almost to zero) in order to get a 10kHz signal up to the right level. I'm wondering if I need to do what smurfer77 is doing and basically rebuild the playback amp board and the RWR-053 board?

 

[smurfer77]

If I had the machine in front of me I could probably probe around with the multimeter and look for something amiss in terms of transistor operation. But without the machine it's hard for me to say what to do next. The EQ adjustments you are describing seem extreme and imply something is not right with the boards or heads.

What I suggest is post more of the details from your experiences that you described to me, into your thread. I suspect the pros might be able to point you into a more focused direction than I could (I'm quite new to tape machines). In particular GPS16, who replied to your thread, has pointed me in the correct direction a couple of times. He knows his stuff. If you don't get good replies on the thread, I will engage my brain a bit harder and try to see what I can come up with beyond a full rebuild

Cheers
David

 

[GPS16]

Hi,
when you say not recording, can you describe what you do get?! Is it silence, distorted, low level, crackly, hissy..... What you get will give pointers as to where to start. From your previous posts, it sounds like you are losing the bias so it will be low level and distorted with a lack of bass.

GPS16

 

[soundmig]

GPS16 ... thanks for chiming in. Since I changed the 1213's out for 2383's the left channel is now basically nothing. I do hear some pretty low level hiss/hum which I attribute to the electronics in the playback chain?

The right channel is now pretty good. Not perfect, but it records and plays back (on test tones) basically the same as the alignment tape. There is a nice strong signal with only some very slight variation in amplitude +/- 0.5 dB if that.

 

[GPS16]

Hi,
if you look at the circuit diagram there are 2 test points just after the equalisation PCB. There should be the record signal there when the machine is in record mode. If the there is no signal there, there is nothing to record. Following this there is a tuned circuit which stops the Bias going backwards into the Record circuits. Then there is the record relay. This relay either shorts the record audio signal to ground during none recording functions or lets the record audio through to add to the bias from VR303. The 6.2k resistor keeps the bias Osc loaded when the channel is not used. If this was not done all Bias levels would change when recording on one channel only.

If you have a scope you should be seeing audio on TP301, Bias on one side of R319 and Audio Plus Bias on the other side of R319. The next stop following this is the record head itself. The bias signal should be a stable amplitude sinewave of 125 kHz.

Keep us posted.

GPS16

 

[soundmig]

Thanks GPS16. I'm out of town at the moment, but will try to figure out how to do those "tests" you describe when I get back. Some of what you said is "greek" to me (and I'm not greek), but once I poke around a bit I have a feeling it will make sense.

I know the record head works as it was all working on that channel (not perfectly but working) after I replaced the 1344 transistor. After I replaced the 1213 transistor is when it stopped working. Anyway, I'll poke around when I get back in town and see what I can come up with Really appreciate your thoughts and direction. Mike G.

 

[GPS16]

Hi,

do you have access to a scope? For this being able to "see" the Bias signal would be of great benefit. There are alot of switches associated with the Bias/Rec circuits to guard against rogue operation which could destroy tapes being played. If you have any further questions that you don't want on public display, PM me. I'm sure we could sort things out.

GPS16

 

[soundmig]

GPS16 ... I have a scope that is a "sample scope" on my Audio Toolbox unit. It's from the 2001 time frame, so its a bit crude - but it works. I'm not sure how I would use it to find things like bias signals, but I believe you could help me figure that out. The inputs on the audio toolbox are RCA or XLR, so I could make up adapters that go from RCA to two bare wires so that I could hook up some probes? The audio toolbox can analyze both channels at one time or just a single channel and it can display as a scope or as a level meter (dBv or dBU) or as an RTA it can also do sweeps and Time Energy Frequency displays. Anyway, It was fast enough and had enough resolution for me to use it to adjust azimuth using a 10KHz single, so I"m pretty sure it will work?

 

[GPS16]

Hi,
had a quick look at the website for audio toolbox. I could not find what the specs were for it though. You should be able to see the audio frequencies OK but not sure about the Bias. Whether you will be able to pick up 125 kHz is another matter. Unfortunately, I suspect that Bias instability may be the problem. Could you look at the spec for the software package is and try and find what it says about upper frequency limit or bandwidth for the scope part of the package.

GPS16

 

[soundmig]

I don't have the literature with me. I do know that at its finest resolution I was able to see the 10kHz waveform, but it was getting small (meaning I could see about 8 or 10 waves across the small screen). At 125kHz I really doubt that I'll be able to see a waveform.

 

[BobbyN]

Hi,
if you look at the circuit diagram there are 2 test points just after the equalisation PCB. There should be the record signal there when the machine is in record mode. If the there is no signal there, there is nothing to record. Following this there is a tuned circuit which stops the Bias going backwards into the Record circuits. Then there is the record relay. This relay either shorts the record audio signal to ground during none recording functions or lets the record audio through to add to the bias from VR303. The 6.2k resistor keeps the bias Osc loaded when the channel is not used. If this was not done all Bias levels would change when recording on one channel only.

If you have a scope you should be seeing audio on TP301, Bias on one side of R319 and Audio Plus Bias on the other side of R319. The next stop following this is the record head itself. The bias signal should be a stable amplitude sinewave of 125 kHz.

Keep us posted.

GPS16[,

While recording with no audio input, should you read that 125 KHZ on the record head?
Bob

While recording with no audio input, should you read that 125 KHZ on the record head?
Bob

 

[soundmig]

Well .... I checked the specs on my TerraSonde Audio Toolbox. Its digital and runs at 48kHz sampling rate (built in 2001), so it only "see's" and measures up to around 20kHz. I can't "see" the bias waveform or even use the frequency counter to check to see if the bias is getting to the heads. I'm not sure what else to do at this point. I think I'll put it all back together and use it to listen to some pre-recorded tapes (its sounds pretty darn good despite the imperfections). When i get access to a real O-scope I'll see if I can learn anything else via those test points.

 

[GPS16]

Hi,
at this point, the bias signal needs to be checked for presence and stability. As the problems are mainly on one channel, the problem would seem to be near to the Record relay than any where else. Hence the advice to cycle the record select switches. The switches themselves need to be operated along with the associated relays. As I have said elsewhere in other threads, switches tend to be designed to self clean. But you have to move them to do so.

Have you tried recording a known signal and monitoring the recording whist carefully moving switches. I could tell if switches were getting dirty by moving the button side to side with just a finger tip. You need to be recording as mentioned above as most of the problem switches are in use during record only!!!

GPS16

 

[soundmig]

GPS16 ... I've listened in "tape" mode while recording (a test tone) on that channel (left) and I can hear (off of the tape) the bump of the relay when I turn the record mode on and off on that channel. So, I think that the relay is working. I don't hear the test tone (clearly visible and audible on "source") when listening to the "tape" with the record mode on or off on that channel. It did work before I changed out the 1213 transistors. Could I have a bad "new" transistor?

 

[SPL db]

I just love troubleshooting threads... carry on!

 

[smurfer77]

following with interest...

 

[GPS16]

Hi,

when you turn on and off the record switch for the left channel the change in state of the record relay should be audible. As mentioned before it is delayed to allow the Bias Osc to die away before the relay cuts off. This is to stop the Bias "magnetising" the record and erase heads when the record mode is terminated. If the record relay is not moving and is in the "off" state the record signal is shorted to ground via the relay. The operation of the Left channel relay must be properly verified.

For a start, with the Right Channel Record set to off, does the record (Red) LED follow the Left Channel when switched on and off (the machine should be running in record mode with the tape moving). If the LED follows this switch then the supply to operate the relay is present at the anode of D301 (they are drawn the wrong way round on the diagram, the right relay controls the left channel). If the LED goes on and off, the relay should click with it but there is a delay. change the state of the Record Left switch slowly to allow the relay to change state. Waiting a second should be ample!!!

Does the relay work??

GPS16